In the packaging of a waveguide system it is sometimes necessary to change the axial orientation of the waveguide by 90 degrees along the length of a waveguide run. For example, the axial orientation of the waveguide may be required to change from an H-plane orientation to an E-plane orientation or the other way around. For a linearly-polarized antenna, an E-plane is the plane containing the electric field vector in the direction of maximum radiation. An H-plane is the plane containing the magnetic field vector in the direction of maximum radiation. The magnetizing field or H-plane is orthogonal to the E-plane.
The electric field or E-plane determines the polarization and orientation of the radio wave. For a vertically-polarized antenna, the E-plane usually coincides with the vertical/elevation plane and the H-plane coincides with the horizontal/azimuth plane. For a horizontally-polarized antenna, the E-plane usually coincides with the horizontal/azimuth plane and the H-plane coincides with the vertical/elevation plane.
Some systems require the rotation of the electro-magnetic fields from an H-plane orientation to an E-plane orientation. A twist or rotation of the E-field is done by a waveguide that physically forces the rotation of the orientation of the E-field (and H-field) by 90 degrees as the electro-magnetic (EM) radiation propagates along the length of the waveguide. A waveguide that physically forces the rotation of the E-field orientation requires a relatively long waveguide length.
Some systems, such as a power dividing network for an antenna array, require the rotation from an H-plane orientation to an E-plane orientation to occur over a very short distance so the twist (rotation of the E-field) occurs in the shortest length possible. Some shorter length twists are currently available. In one example, a quarter wavelength section orientated at 45 degrees is placed between the orthogonal waveguides. In another example, a resonant iris orientated at 45 degrees is placed between the orthogonal waveguides. A resonant iris can take various forms but is typically an approximately half wavelength slot at the desired frequency, separating the input and output sections of waveguide. Both the quarter wavelength section and the resonant iris have a narrow bandwidth and are sensitive to bandwidth. A resonant iris is also sensitive to machining tolerances due to narrow gaps in the iris.